As is known in the art, radar systems, such as pulse radar systems, are used to determine the range and/or relative velocity of an object. Radar pulses are transmitted at a rate referred to as the pulse repetition frequency (PRF). The time interval between successive pulses is referred to as the pulse repetition time (PRT). During a predetermined time after pulse transmission, radar return signals are sampled, or range gated, by the radar signal. That is, based on the difference in time between pulse transmission and the time which the sample is taken, each one of the samples corresponds to a range, or distance, between the radar system and the object producing the sampled return. The process is referred to as range gating, where each time a sample is taken represents a range cell, or gate, of the return produced by the object at the range corresponding to the time at which the sample is taken.
In order to determine the velocity of the object, the radar returns from a plurality of transmitted radar pulses are processed. More particularly, in a pulse Doppler radar each set of radar returns from a plurality of consecutively transmitted radar pulses is referred to as a dwell. The radar system produces a plurality of consecutive dwells. For each dwell, the radar system determines the average frequency of an object at one of a plurality of contiguous range gates. Fine velocity resolution generally requires a large number of radar returns per dwell (i.e., a relatively large data collection period). In a pulse radar system without Doppler processing, the time difference in the return delay of pulses can be compared to determine relative velocity.
In the event two or more pulse radar systems are located in proximity to each other and operate in or near the same frequency band, crippling mutual interference can occur. The interference between systems is characterized by pulses from one system appearing repeatedly in the same range gate of another system, usually at irregular intervals. Although the systems use different pulse repetition times (PRTs), the pulses from one system often line up in time with the pulses from another system in the other system's receiver. To each individual radar receiver, this appears as either an impulse added to the incoming data stream or a suppression at one point in time in the incoming data steam.
The effects of these asynchronous pulses are detrimental because they often can cover up any target signals preventing the target from being detected or causing false alarms. In a pulse Doppler radar this happens in the Fast Fourier Transform (FFT) of the received signal. In a pulse radar without Doppler processing, this happens due to ringing of the asynchronous pulses through the moving target indicator (MTI) filter. The PRT sets are typically chosen to maximize the time interval between interfering pulses, but it is impossible to mitigate the interfering pulses entirely.
In the past, asynchronous pulse detectors have been developed to address problems associated with interfering pulse radar systems. However, the asynchronous pulse detectors did have some shortcomings. For example, such asynchronous pulse detectors replaced asynchronous pulses with previous data samples. This could cause problems in radar receiver designs where clutter was not tuned to zero, for instance. The replacement data was taken from the input of the asynchronous pulse detector. This meant that during a false alarm, an asynchronous pulse could be inserted into the data stream rather than removed. Moreover, the asynchronous pulse detector could not remove more than one consecutive asynchronous pulse at a time, and asynchronous pulses often appear consecutively in groups of more than one.
In view of the aforementioned shortcomings associated with conventional asynchronous pulse detectors in pulse radar systems, there is a strong need in the art for an improved asynchronous pulse detector for detecting and removing asynchronous pulses. More specifically, there is a strong need in the art for an asynchronous pulse detector which is not as susceptible to inserting an asynchronous pulse into the data stream rather than removing it in the case of a false alarm. Moreover, there is a strong need in the art for an asynchronous pulse detector capable of removing consecutive asynchronous pulses appearing even in groups of two or three from the data stream.